1) Field of the Invention
This invention relates to a control method for an automatic transmission, especially to a method for the control of servo hydraulic pressure adapted to have frictional engagement elements engaged or released for changing the gear position of an automatic transmission.
2) Description of the Related Art
An automatic transmission mounted on a vehicle is designed to provide a multiplicity of speed stages with a view toward improving the running performance, gas mileage, riding comfort and the like to the vehicle. However, increases in dimensions and weight which result from the provision of such a multiplicity of speed stages impair the mountability of the automatic transmission on the vehicle and reduce the gas mileage. To provide automatic transmissions with a multiplicity of speed stages, there is therefore a need for further dimensional and weight reduction and also for further structural simplification. In view of the foregoing, the assignee of the present application has already developed a five-speed automatic transmission in which an inlet-side overdrive planetary gear is combined with a 4-forward/1-reverse primary shifting unit composed of an in-line train of three planetary gears. The gear train of the primary shifting unit, however, requires engagement of friction elements of different brakes upon shifting from a 2nd speed to a 3rd speed. This engagement tends to produce a shift shock. In a method commonly employed to avoid such a shift shock, a one-way clutch is inserted in series with frictional engagement elements which take part in the engagement. Addition of such a structural element is however contradictory with the above-described objective of dimensional and weight reduction. It is desirable to avoid the addition of such a structural element if another measure is available.
Conventional methods of controlling servo pressure, which involve engagement of different frictional engagement elements as described above, include the use of a timing valve. In these conventional methods, the timing valve is actuated when the servo pressure of servo means, adapted to operate a friction element on an engaging side, has reached a predetermined level or higher. A large-diameter orifice is then opened to promptly drain the servo pressure on the releasing side. This makes it possible to control the timings of the engagement and release. Further, as is disclosed in Japanese Patent Application Laid-Open No. SHO 64-65354, both throttle pressure and engaging pressure are applied in a mutually opposing relation against a valve spool so that the change-over of an the orifice for controlling drain pressure is performed at a timing corresponding to the engine load.
In the former technique, the timing valve is designed to change over against spring load at a predetermined constant pressure. The pressure employed to operate the timing valve is throttle pressure which also controls the line pressure and the engaging pressure. Good change-over performance cannot therefore be achieved under certain shift conditions. A shift shock occurs due to tie-up of a brake especially when the accelerator pedal is released in the course of a shift and the input torque is changed accordingly. Since the timing of change-over of a timing valve is determined by the initial setting, it is impossible to make a correction against a deviation in the coefficient of friction of a friction element, time-dependent reduction of engine torque, or the like. Further, it is also impossible to cope with variations in temperature or a change in throttle opening during a shift because the control of the change-over timing is effected by switching drain orifices. No matter which conventional technique is used, it is extremely difficult to restore drain pressure once drainage is started. This makes it impossible to promptly correct the timing of change-over responsive to variations in the input torque.